The present invention relates to a text processing device having a display unit for representing the data corresponding to the inputted codes as well as a printing unit for printing the inputted data, while capable of replacing the codes stored in a video RAM(Random Access Memory, named VRAM, hereinafter) storing the codes corresponding to the data represented on the display unit along with a driving operation for driving, for example, a carriage and the like without an abnormality caused by the driving operation simultaneously executed with the above replacing operation.
Thus far, for example, in a text processing device having a function which edits inputted data through a keyboard and the like on a raster type CRT(Cathode Ray Tube) along with displaying them and prints the inputted data which has been edited by using a printing unit, simultaneously to control various drive motors which structure the printing unit and to replace the contents of the VRAM which structures the display control circuit of the raster type CRT for displaying the inputted data through the keyboard, two CPU (Central Processing Unit) devices, one of which controls the printing unit and the other controls the display control circuit, have been used.
However, in this prior art, because the two CPU devices for controlling both the printing unit and the display have to be used, it was difficult to reduce a product cost.
To solve the problem, it is possible to control the above two processes by using one CPU device. However, the following problems arise.
That is, in a text processing device such as a word processor, an operation which displays data on CRT along with printing the data is required. At the time, a process which replaces the contents of VRAM and a process which updates output phases of various drive motors of the printing unit are executed. However, because the contents of VRAM is replaced, the timings of the pulse update process of the drive motors may be delayed, the pulse widths may deviate and the drive motors may get disordered, i.e., the drive motors may become a so-called step out.
In addition, the VRAM replace process is executed during a vertical retrace period (hereinafter named the retrace period). To prevent the pulse signals from the above interfering, if the drive motors are controlled in the highest priority than others, because the major purpose of the text processing apparatus is to print text, it is necessary to wait for the VRAM replace process until the retrace period passes. If the wait time becomes long, the data may not be replaced in the retrace period. In this case, after the retrace period, since the VRAM replace process is continuously executed, the CRT screen flickers.